The present invention relates to the arrangement of gearwheels in a dual clutch transmission, especially a dual clutch transmission for on-road vehicles.
In a dual clutch transmission with coaxial input and output shafts, a small main pilot bearing is arranged between the centre shaft and the first input shaft. The pilot bearing is subjected to large axial forces in some gear speeds. The large axial forces originate from the helical gears, which when transferring torque are subjected to a gear mesh force. These large axial forces are a problem especially in gear speeds when there will be no relative rotation in the main pilot bearing. Large axial forces and no relative rotation are very unfavourable operating conditions for a small bearing. Fretting wear may occur, which will lead to premature damage of the bearing.
U.S. Pat. No. 5,181,431 discloses example of prior art.
A solution o the problem would be to increase the size of the bearing. There is however no room available for a larger bearing.
It is desirable to provide an inventive gear arrangement in a dual-clutch transmission, said gear arrangement aims to decrease the load upon the pilot bearing in situations where there is no relative rotation in the bearing.
According to an aspect of the invention, a dual clutch transmission comprises a first and a second input shaft provided with a first and second input means respectively, a centre shaft, a countershaft and a first and a second primary gear step. The first and second input means are normally a first and a second clutch. The first and second input shafts are coaxially arranged with the centre shaft and can transfer torque from said first and second input means to said countershaft via said first and said second primary gear step. This is a well known configuration, used in several dual clutch transmissions.
The first primary gear step comprises a first input gearwheel and a first driven gearwheel, wherein said first input gearwheel is rotationally fixed with said first input shaft. According to the invention, the first input gearwheel is arranged upon said centre shaft, instead of upon the first input shaft as previously known.
By arranging the first input gearwheel upon the centre shaft instead of the first input shaft, as previously known, axial gear mesh forces that act on the first primary gearwheel will be taken up and carried by the main shaft, whereby the pilot bearing is subjected to less axial load.
It is preferred that the first input shaft is connected to the first input gearwheel through a connection allowing an axial play, whereby the connection not is able to transfer any axial load. The connection comprises a first part belonging to the input shaft and a second part belonging to the centre shaft. One such preferred connection is a spline coupling. A spline coupling could be integrated, whereby it comprises mating clutch teeth on the first input shaft and first primary cut gearwheel 332. The spline coupling could alternatively comprise a bridging element that mates with clutch teeth on the first input shaft and the first primary cut gearwheel. However, the connection is always engaged. An alternative coupling could be axially directed dog clutch teeth.
It is further preferred that said first input shaft arranging said first input gearwheel upon conical roller bearings, which can be subjected to relatively high axial loads.
Alternatively, the first input gearwheel is arranged upon cylindrical roller bearings, which could be preferred in a cost perspective relative conical roller bearings.
It is further advantageous that said first input gearwheel is arranged upon said centre shaft such, that it can be axially displaced between a distanced axial position and a contact axial position upon the centre shaft, but always rotationally fixed to said first input shaft. In the distanced axial position, the first input gearwheel is axially distanced from the first part of said connection and in the contact axial position, an axial contact between the first input gearwheel and the first part of said connection is enabled. In the contact position, axial forces acting upon the input gear wheel can thereby be transferred to the first input shaft, whereby the pilot bearing is further relived from axial load.
It is foreseen that the first input gearwheel is displaced into said contact axial position when it transfers a torque load in a first direction. The axial displacement is due to gear mesh forces acting upon the first input gearwheel.
The first input gearwheel and the first driven gearwheel are provided with helical gear teeth, whereby an axial displacement of the first input gearwheel, from said distanced axial position to the contact axial position, is actuated when the first input gearwheel transfers a torque load to the driven gearwheel.
To achieve the axial sliding of the first input gearwheel it is mounted such upon a first and a second roller bearing that it can slide thereupon. The first input gearwheel is thereby provided with a radially inwards directed protrusion, which is arranged such between said first and second roller bearing that said axial displacement is allowed. The radially inwards directed protrusion thereby has an axial extension which is shorter than the corresponding distance between the first and the second roller bearing.
In a further embodiment of the invention the first input gearwheel is provided with a synchronisation mechanism or a dog clutch enabling a rotational connection between said first input gearwheel and said centre shaft. By closing the synchronisation mechanism or the dog clutch a direct connection between the first input shaft and the centre shaft is achieved.